1. Field of the Disclosure
The present disclosure relates to laminated composite structure, and more particularly relates a composite laminated structure having at least one reinforcing pin. The disclosure also relates to a method for reinforcing a laminated composite structure.
2. Description of the Related Art
It is well known in the field of composite structures that two-dimensional composite laminates are relatively weak in the so-called “through thickness” or z-direction. At the interfaces between adjacent laminae in such structures there is a high tendency for the plies to debond or delaminate. This problem has been found to be particularly prevalent in composite laminates in which adjacent laminae have constituent fibres which are not aligned. This problem has therefore led to the development of various types of “through-thickness-reinforcement”, commonly known by the acronym TTR, such as z-pinning, stitching, tufting, and three-dimensional weaving, all of which will be well known to those of skill in the art.
Z-pinning is a technique whereby a plurality of small pins are inserted into an uncured laminated composite structure such that the pins extend between adjacent laminae in the structure, which provides a significant improvement in through-thickness strength and resistance to delamination of a composite structure. The most common type of pin insertion process is currently to use an ultra-sonic hammer and pre-formed arrays of pins. The pins are held in spaced relation to one another in a foam carrier. The foam carrier is positioned against a surface of the composite structure and the ultrasonic hammer is used to press the pins through and out of the foam and into the composite structure. This z-pinning technique requires the pins to have chamfered tips and for their outer surfaces to be relatively smooth. As the ultrasonic hammer urges the pins into the composite structure, it induces high frequency vibrations in them. The vibrating chamfered tips of the pins locally heat up and soften the matrix resin of the composite material, allowing the pins to penetrate the composite material with minimal disruption to the fibres. This process is efficient and has therefore become widely used for large-scale production of TTR composite structures. However, as indicated above, it places particular design requirements on the form of the pins which must be used.
Fibrous composite pins are widely used for z-pinning in order to reinforce composite structures. However, there are significant disadvantages in using this type of material for the pins. For example fibrous composite pins have a tendency to split during aggressive penetration of the structure to be reinforced, the material of the pins is rather brittle and can result in premature failure of the pins, and the pins will have a low transverse modulus and strength which reduces their effectiveness in shear loading of the reinforced structure.
For these reasons it has been proposed previously to use metal pins as an alternative. However, metal pins have heretofore suffered from a significant disadvantage themselves arising from the fact that their coefficient of thermal expansion is poorly matched to that of the composite materials which there are intended to reinforce. This can result in early debonding of the pins from the surrounding matrix material of the composite during the manufacturing process, thereby degrading the pins' reinforcing performance.